1. Field of the Invention
This invention relates to multiple-exposure holographic interferometry and more particularly to a method and apparatus for the recording and readout of multiple exposure holograms utilizing heterodyne interferometry.
2. Description of the Prior Art
The application of multiple reference wave holographic interferometry to flow visualization and for determining deformation in objects has been documented in the literature (see, for example, an article by J. D. Trolinger entitled "Application of Generalized Phase Control During Reconstruction to Flow Visualization Holography" which appeared in Applied Optics, Mar. 15, 1979). This technique has been used as a diagnostic tool to study various flow fields including laser gas flows and the interactions of flows with airfoils. Typically, after a double pulse hologram is exposed and developed, an interferogram is produced from the two reconstructed wavefronts. Next, the interference fringes are digitized by determining the fringe center coordinates. For fringes with widely varying spacings across the interferogram, the digitization hardware can have difficulties locating fringe centers since video or solid-state image sensors have a fixed pixel size. This is particularly difficult for fringes with high spatial frequencies. Also, the digitization software tends to be complex since fringe centers must be located in the presence of varying contrast and in the presence of noise (speckle and diffraction artifacts). Another complication is the unraveling of fringe contours. For fringes that cannot be recognized by the software, an operator may have to manually determine the fringe centers.
U.S. Pat. No. 4,428,675, issued to Witherow, teaches a method for double-exposure holographic interferometry (test and reference beam pulses being formed) utilizing two lasers that sequentially output optical pulses with varying temporal spacing (10 nanoseconds or greater). The optical system is such that both reference beam pulses follow the same optical path. Thus, this method will not allow the two holograms to be addressed separately for readout.
U.S. Pat. No. 3,934,461, issued to Heflinger, teaches a method for recording multiple-exposure holograms with a double-pulsed laser using a single test beam path. In the reference path, both pulses follow the same beam path. The holograms are reconstructed with a continuous laser which is directed along the reference path. A pivoting mirror is utilized to direct either the pulsed laser beam toward a film path to record the holograms or the continuous laser to reconstruct the holograms toward the film plane. With the addition of the continuous laser, the hologram can be read out using the same optical system. Coincident recording reference beams for both holograms, as in Witherow, prevent the use of two reference beams during readout.
A method for producing two pulses from a single laser used in a double-pulsed holographic interferometer is described by W. T. Armstrong and P. R. Forman in the article entitled "Double-Pulsed Time Differential Interferometry" which appeared in Applied Optics, January 1977. In that paper the laser produces two pulses of orthogonal polarization and of varying time separation (100 nanoseconds or greater). The output beam is split into a test and reference beam, with the reference beam then being separated into two reference beams with a polarization sensitive Rochon prism. The Rochon prism introduces several milliradians of angular separation between the two beams.
The addition of this small tilt between the two recording reference beams allows tilt fringes to be viewed between the two holograms upon reconstruction. Such a hologram is read out with a single reference beam since the two recording reference beams do not have sufficiently large angular spacing. This prevents the recorded wavefronts stored in the holograms from being individually addressed with a phase or frequency modulated reference beam to allow for electronic phase readout.